Pursuant to Federal Aviation Regulation (FAR) Section 25.981(b) and European Aviation Safety Agency Certification Specification (EASA CS) Section 25.981(b)(2)(b)(3) amendment 6,new commercial aircraft are required to have a low flammability exposure. This has led to the development of inert gas generating systems (also known as OBIGGS (on-board inert gas generating systems), FRS (flammability reduction system), NGS (nitrogen generating system), FTIS (fuel tank inerting system)) on commercial aircraft. In addition, many military aircraft have incorporated fuel tank inerting systems into their designs. Such fuel tank inerting systems supply an inert gas, such as nitrogen enriched air (NEA), to purge fuel tanks and effectively reduce oxygen concentration levels therein. The component of such fuel tank inerting systems that enriches nitrogen is generally known as the gas separation assembly, or more particularly, as the air separation module (ASM). The gas separation assembly or ASM is used to generate NEA. Known gas separation assemblies or ASMs typically includes a fiber bundle 52 comprised of hollow fiber membranes 54 held by tubesheets 46 on each end and encapsulated by a shell or housing 32 (see FIG. 2A). Known gas separation assemblies or ASMs expose the hollow fiber membranes 54 by cutting off one side of the tubesheet 46 to expose the hollow fiber membranes 54 and openings 50 on the face 48 of the tubesheet 46.
Known tubesheets may be flat, may not be easily reinforced, and may typically be the life-limiting component of the gas separation assembly or ASM. In known gas separation assembly or ASM designs, feed gas, such as pressurized air, flows into or enters the gas separation assembly or ASM on one side of the tubesheet. Such design uses the tubesheet as a pressure boundary and can put stress or pressure on the tubesheet, which can reduce the service life of the tubesheet, and in turn, reduce the service life of the gas separation assembly or ASM. Moreover, the exposed hollow fiber membranes may be embedded in an epoxy matrix, and pressurizing the face or flat end of the tubesheet having the exposed hollow fiber membranes that have been embedded in the epoxy matrix may cause the epoxy matrix to crack and/or creep or separate from the fiber bundle, thus causing an aperture or opening for depressurization, which can lead to failure of the gas separation assembly or ASM. Further, due to material properties and design of known tubesheets, the gas separation assembly or ASM may not meet its expected service life at a desired system temperature.
To increase the service life of the gas separation assembly or ASM, known inerting systems have lowered the operating temperature of the system. The tubesheet material may have greater strength at lower temperatures but the lower temperature can reduce the inerting system's performance and may drive the need for additional gas separation assembly or ASM weight or pressure-boosting components. This is because higher temperatures may increase the efficiency of the separation of nitrogen and oxygen. Moreover, some known systems, commonly known as shell-side feed systems, reverse the flow of the feed gas, such as air, which allows the gas separation assembly or ASM shell or housing to provide support to the tubesheet. However, this can also reduce the performance of the gas separation assembly or ASM.
Accordingly, there is a need in the art for a gas separation assembly or ASM and method that provide advantages over known assemblies, systems, and methods.